Micro-Electro-Mechanical System (MEMS) devices may be used in various applications such as micro-phones, accelerometers, inkjet printers, etc. A commonly used type of MEMS devices includes a MEMS capacitor that has a movable element (sometimes referred to as a proof mass) as a capacitor plate, and a fixed element as the other capacitor plate. The movement of the movable element causes the change in the capacitance of the capacitor. The change in the capacitance may be converted into the change in an electrical signal, and hence the MEMS device may be used as a micro-phone, an accelerometer, or the like.
The movable elements of MEMS devices may move in air cavities when the MEMS devices are operated. The resistance of the air (and/or out-gassed gases from the respective chip) in the air cavities to then movable elements is preferably small. The resistance is related to the pressure in the air cavities. To reduce the resistance, the pressure in the air cavities should be small. The reduction of the air pressure may be achieved by increasing the volume of the cavities. This approach, however, encounters process difficulties.
The air cavities may be formed in a MEMS wafer, in which the MEMS devices are located. When the cavities are enlarged, the bonding area that can be used for the MEMS wafer to be bonded to another wafer (such as a cap wafer), however, is reduced since the bond is formed on the portions of the MEMS wafer where no cavity is formed. The reduction in the bonding area may cause the reliability of the bond to be sacrificed.
The air cavities may also be formed in a cap wafer that is used to protect the MEMS wafer. The cap wafer, however, needs to have some portions removed, so that the bond pads in the MEMS wafer may be exposed in order to perform wire bonding on the bond pads. The removal of these portions of the cap wafer and the formation of the cavities in the cap wafer require separate lithography masks, and the manufacturing cost is high.